1. Technical Field
The disclosure relates to a lithium ion secondary battery.
2. Description of Related Art
Japanese Unexamined Patent Application Publication No. 2008-277106 (JP 2008-277106 A) discloses a lithium ion secondary battery including a pressure-actuated current interrupt device, a positive electrode mixture including lithium carbonate, and an electrolytic solution including biphenyl (hereinafter, in some cases, abbreviated as “battery”).
Overcharging is one of the abnormal modes of batteries. For example, when abnormality is caused in the control of a charging apparatus, charging batteries beyond their full charging capacity, that is, overcharging occurs.
Devices that safely bring battery functions to a stop using the generation of gas in batteries during overcharging (hereinafter, referred to as “pressure-actuated safety devices”) are proposed. Pressure-actuated safety devices convert pressures attributed to the generation of gas to, for example, mechanical operations such as blocking of circuits. When pressure-actuated safety devices are exposed to high voltages in order to accelerate the operation of the devices during overcharging, overcharging additives generating gases are used.
As overcharging additives that are added to positive electrodes, lithium carbonate (Li2CO3) is proposed. During overcharging in temperature environments of 60° C. or higher, lithium carbonate actively decomposes and generates a large amount of carbon dioxide (CO2) gas. However, in temperature environments of lower than 60° C., the amount of CO2 gas generated is smaller than usual. This is considered to be because, in temperature environments of lower than 60° C., the decomposition reaction of lithium carbonate is not active.
As overcharging additives that are added to electrolytic solutions, benzene derivatives (for example, biphenyl and the like) having a lower oxidation potential than electrolytic solution solvents (hereinafter, also simply referred to as “solvents”) are proposed. During overcharging in temperature environments of lower than 60° C., benzene derivatives cause polymerization reactions in positive electrodes and generate protons (H+). Protons migrate to negative electrodes, and the protons are reduced in the negative electrodes. Therefore, a large amount of hydrogen (H2) gas is generated. However, in temperature environments of 60° C. or higher, the amount of H2 gas generated is smaller than usual. This is considered to be because, in temperature environments of 60° C. or higher, a phenomenon in which benzene derivatives are oxidized and turned into cations, the cations migrate to negative electrodes, reduce the negative electrodes, and then return to positive electrodes (so-called “redox shuttle phenomenon”) becomes dominant.
FIG. 1 is a schematic graph illustrating a relationship between the amount of CO2 gas generated by lithium carbonate and the amount of H2 gas generated by benzene derivatives, and temperature. As illustrated in FIG. 1, lithium carbonate and benzene derivatives have mutually different temperature environments in which the amount of gas generated increases on both sides of a temperature near 60° C.
Therefore, in order to accelerate the generation of gas in wide ranges of temperature environments, that is, both temperature environments of lower than 60° C. and temperature environments of 60° C. or higher, the use of a combination of lithium carbonate and benzene derivatives is considered. However, in this case, it is considered that the securement of the amount of gas generated in temperature environments of lower than 60° C. is difficult. That is, lithium carbonate is weakly alkaline. When alkaline components coexist, protons generated by the polymerization reactions of benzene derivatives are trapped by the alkaline components. Therefore, it is considered that protons are not capable of reaching negative electrodes and the amount of H2 gas generated is smaller than usual.